Conventionally, in a system to which a wireless communication system including a long term evolution (LTE) is applied, a base station transmits a control signal for allocating a resource of an uplink direction to a mobile station prior to data reception when the mobile station transmits data to the base station. The mobile station transmits the data and a reference signal (RS) to the base station upon reception of the control signal. The RS is transmitted as one symbol in the same time slot as the data through a physical uplink shared channel (PUSCH). The base station performs channel estimation based on the RS, and demodulates the data received through the PUSCH using an estimation result. The RS is called demodulation RS (DM-RS) because of being used for demodulation of received data.
For example, in the LTE, a Zadoff-Chu (ZC) group is used as a group of the DM-RS, and identification parameters thereof include a group number, a sequence number, a cyclic shift amount, and the like. Among these parameters, the group number and the sequence number in each time slot can be uniquely identified from a cell ID. The cyclic shift amount is specified by the above-described control signal (uplink grant) immediately before the mobile station transmits the data. Note that, if the interval of the cyclic shift is made too small, a plurality of signals may overlap with one another when a large multipath having a large time difference is generated. Therefore, as an upper limit value of the cyclic shift amount, “8” is set, for example, and this value becomes the number of allocatable resources by the base station for transmission of the DM-RS.
Non Patent Document 1: 3GPP TS36.211 V10.2.0 (2011-06)
Non Patent Document 2: 3GPP TR36.814 V9.0.0 (2010-03)
However, the above-described technology has following problems with an increase in the number of mobile stations that perform communication using the same resource in the same cell. That is, in a wireless communication system in which a plurality of remote radio heads (RRHs) is disposed in a macrocell, an antenna of the base station that forms the macrocell and a plurality of antennas included in the RRHs coexist. Therefore, the number of antennas arranged in one cell is increased. Accordingly, the number of mobile stations that perform communication using the same resource (a time and a frequency) in the same cell is increased.
Meanwhile, the number of resources allocated by the base station for transmission of the DM-RS usually requests a total value of the number of layers of multiple input multiple output (MIMO) of the mobile stations that simultaneously perform communication. Therefore, the above-described value “8” may be insufficient as the resources when the mobile stations transmit the DM-RSs to the base station. Further, even when the resources are sufficient, a probability that the same resource is allocated to (conflicts in) a plurality of mobile stations is increased, and there is a possibility that channel estimation accuracy with respect to each mobile station is decreased in the base station that has received the DM-RSs from the plurality of mobile stations using the resource. Since the base station demodulates the data received through the PUSCH using the channel estimation result, the decrease in the channel estimation accuracy may be a cause that impedes normal demodulation of the received data.